1. Field of the Invention
This invention relates to redundancy systems for data communications wherein two duplicate communication channels each having transmission and reception circuits are provided for reliability. Such redundancy systems frequently are used in process control arrangements to permit the controlled process to continue despite equipment breakdown.
2. Description of the Prior Art
Prior art techniques use various different redundancy systems to attain high-reliability data communications. Such redundancy systems typically are arranged either as parallel redundancy systems or as standby redundancy systems. The former is characterized in that the two duplicate systems are operated in parallel and when any discrepancy arises between data transmitted by the two systems, the failed side is isolated for services. In standby redundancy systems, one of the two systems serves as a standby for the other. Both such redundancy systems, however, are not fully satisfactory. The parallel redundancy system must have two complete pairs of control and error detection circuits, entailing a significant cost. In the standby redundancy system, a failure in the standby side remains undetected until the master side fails because the standby becomes active only in an emergency.